Golf ball

ABSTRACT

A golf ball can include a core, mid layer, and a cover. The golf ball can satisfy the following mathematical formulas (1) and (2), 
     
       
         
           
             
               
                 
                   
                     Fw 
                   
                   / 
                   
                     Fwn 
                   
                 
               
             
             ≤ 
             0.98 
           
         
       
     
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             Fw 
                           
                           / 
                           
                             Fwn 
                           
                         
                       
                     
                   
                   2 
                 
               
               / 
               
                 
                   
                     
                       
                         Fp 
                       
                       / 
                       
                         Fpn 
                       
                     
                   
                 
                 ≤ 
                 0.98 
               
             
           
         
       
     
      , wherein Fw represents a maximum vertical force of the golf ball measured under a first condition, Fwn represents a maximum vertical force of a standard ball measured under the first condition, Fp represents a maximum vertical force of the golf ball measured under a second condition, and Fpn represents a maximum vertical force of the standard ball measured under the second condition.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Patent Application No. 2021-134160 filed in JAPAN on Aug. 19, 2021. The entire disclosures of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND Field

The present specification discloses golf balls each having a core, a mid layer, and a cover.

Description of the Related Art

An interest to golf players concerning golf balls is flight performance. Golf players prefer a golf ball with which a large flight distance is achieved. Golf players also place importance on feel at impact of a golf ball. Generally, players prefer soft feel at impact.

A typical golf ball has a core, a mid layer, and a cover. JP2020-171623 discloses improvements on a core, a mid layer, and a cover.

Golf players place importance on a flight distance upon a shot with a driver. Players further place importance on a flight distance upon a shot with a middle iron.

Golf players place importance on feel at impact upon a shot with a driver. Players further place importance on feel at impact upon putting.

It is an intention of the applicant to provide a golf ball having excellent flight performance upon a shot with a driver, excellent flight performance upon a shot with a middle iron, excellent feel at impact upon a shot with a driver, and excellent feel at impact upon putting.

SUMMARY

A preferred golf ball includes a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, and satisfies the following mathematical formulas (1) and (2),

(Fw/Fwn)  ≤ 0.98,

(Fw/Fwn)²/(Fp/Fpn) ≤ 0.98,

wherein Fw represents a maximum vertical force of the golf ball measured under a first condition described later, Fwn represents a maximum vertical force of a standard ball measured under the first condition, Fp represents a maximum vertical force of the golf ball measured under a second condition described later, and Fpn represents a maximum vertical force of the standard ball measured under the second condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view schematically showing a golf ball according to an embodiment.

FIG. 2 is a front view showing a driver for evaluating the golf ball in FIG. 1 .

FIG. 3 is a cross-sectional view taken along a line III-III in FIG. 2 .

FIG. 4 is a plan view showing a putter for evaluating the golf ball in FIG. 1 .

FIG. 5 is a cross-sectional view taken along a line V-V in FIG. 4 .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail preferred embodiments with appropriate reference to the drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, a mid layer 6 positioned outside the core 4, a cover 8 positioned outside the mid layer 6, an inner paint layer 10 positioned outside the cover 8, and an outer paint layer 12 positioned outside the inner paint layer 10. The number of paint layers is two. The outer paint layer 12 is a paint layer positioned on the outermost side. The number of paint layers may be one or may be three or more. The golf ball 2 has a plurality of dimples 14 on the surface thereof. Of the surface of the golf ball 2, a part other than the dimples 14 is a land 16. The golf ball 2 has a mark layer on the external side of the cover 8.

The golf ball 2 preferably has a diameter of not less than 40 mm and not greater than 45 mm. From the viewpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably not less than 42.67 mm. From the viewpoint of suppression of air resistance, the diameter is more preferably not greater than 44 mm and particularly preferably not greater than 42.80 mm.

The golf ball 2 preferably has a weight of not less than 40 g and not greater than 50 g. From the viewpoint of attainment of great inertia, the weight is more preferably not less than 44 g and particularly preferably not less than 45.00 g. From the viewpoint of conformity to the rules established by the USGA, the weight is particularly preferably not greater than 45.93 g.

The core 4 is formed by crosslinking a rubber composition. Examples of preferable base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylenepropylene-diene copolymers, and natural rubbers. From the viewpoint of resilience performance of the core 4, polybutadienes are preferable. When a polybutadiene and another rubber are used in combination, it is preferred if the polybutadiene is a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably not less than 50% by weight and particularly preferably not less than 80% by weight. A polybutadiene in which the proportion of cis-1,4 bonds is not less than 80% is particularly preferable.

The rubber composition of the core 4 preferably includes a co-crosslinking agent. Preferable co-crosslinking agents from the viewpoint of durability and resilience performance of the golf ball 2 are monovalent or bivalent metal salts of an α, β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Examples of preferable co-crosslinking agents include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesium methacrylate. From the viewpoint of resilience performance of the core 4, zinc acrylate and zinc methacrylate are particularly preferable.

The rubber composition may include a metal oxide and an α, β-unsaturated carboxylic acid having 2 to 8 carbon atoms. They both react with each other in the rubber composition to obtain a salt. The salt serves as a co-crosslinking agent. Examples of preferable α, β-unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of preferable metal oxides include zinc oxide and magnesium oxide.

The amount of the co-crosslinking agent per 100 parts by weight of the base rubber is preferably not less than 10 parts by weight and not greater than 45 parts by weight. The golf ball 2 in which this amount is not less than 10 parts by weight has excellent resilience performance. From this viewpoint, this amount is more preferably not less than 15 parts by weight and particularly preferably not less than 20 parts by weight. The golf ball 2 in which this amount is not greater than 45 parts by weight has excellent feel at impact. From this viewpoint, this amount is more preferably not greater than 40 parts by weight and particularly preferably not greater than 35 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic peroxide. The organic peroxide serves as a crosslinking initiator. The organic peroxide contributes to the resilience performance of the core 4. Examples of suitable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. An organic peroxide with particularly high versatility is dicumyl peroxide.

The amount of the organic peroxide per 100 parts by weight of the base rubber is preferably not less than 0.1 parts by weight and not greater than 3.0 parts by weight. The golf ball 2 in which this amount is not less than 0.1 parts by weight has excellent resilience performance. From this viewpoint, this amount is more preferably not less than 0.3 parts by weight and particularly preferably not less than 0.5 parts by weight. The golf ball 2 in which this amount is not greater than 3.0 parts by weight has excellent feel at impact. From this viewpoint, this amount is more preferably not greater than 2.5 parts by weight and particularly preferably not greater than 2.0 parts by weight.

Preferably, the rubber composition of the core 4 includes an organic sulfur compound. The organic sulfur compound contributes to the resilience performance of the core 4. Organic sulfur compounds include naphthalenethiol compounds, benzenethiol compounds, and disulfide compounds.

Examples of naphthalenethiol compounds include 1-naphthalenethiol, 2-naphthalenethiol, 4-chloro-1-naphthalenethiol, 4-bromo-1-naphthalenethiol, 1-chloro-2-naphthalenethiol, 1-bromo-2-naphthalenethiol, 1-fluoro-2-naphthalenethiol, 1-cyano-2-naphthalenethiol, and 1-acetyl-2-naphthalenethiol.

Examples of benzenethiol compounds include benzenethiol, 4-chlorobenzenethiol, 3-chlorobenzenethiol, 4-bromobenzenethiol, 3-bromobenzenethiol, 4-fluorobenzenethiol, 4-iodobenzenethiol, 2,5-dichlorobenzenethiol, 3,5-dichlorobenzenethiol, 2,6-dichlorobenzenethiol, 2,5-dibromobenzenethiol, 3,5-dibromobenzenethiol, 2-chloro-5-bromobenzenethiol, 2,4,6-trichlorobenzenethiol, 2,3,4,5,6-pentachlorobenzenethiol, 2,3,4,5,6-pentafluorobenzenethiol, 4-cyanobenzenethiol, 2-cyanobenzenethiol, 4-nitrobenzenethiol, and 2-nitrobenzenethiol.

Examples of disulfide compounds include diphenyl disulfide, bis(4-chlorophenyl)disulfide, bis(3-chlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(3-bromophenyl)disulfide, bis(4-fluorophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(4-cyanophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(3,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(3,5-dibromophenyl)disulfide, bis(2-chloro-5-bromophenyl)disulfide, bis(2-cyano-5-bromophenyl)disulfide, bis(2,4,6-trichlorophenyl)disulfide, bis(2-cyano-4-chloro-6-bromophenyl)disulfide, bis(2,3,5,6-tetrachlorophenyl)disulfide, bis(2,3,4,5,6-pentachlorophenyl)disulfide, and bis(2,3,4,5,6-pentabromophenyl)disulfide.

From the viewpoint of resilience performance, the amount of the organic sulfur compound per 100 parts by weight of the base rubber is preferably not less than 0.1 parts by weight, more preferably not less than 0.2 parts by weight, and particularly preferably not less than 0.3 parts by weight. From the viewpoint of soft feel at impact, this amount is preferably not greater than 1.5 parts by weight, more preferably not greater than 1.0 part by weight, and particularly preferably not greater than 0.8 parts by weight. Two or more organic sulfur compounds may be used in combination.

The rubber composition of the core 4 may include a carboxylic acid or a carboxylate. The carboxylic acid and the carboxylate can contribute to making the hardness distribution of the core 4 appropriate. An example of preferable carboxylic acids is benzoic acid. Examples of preferable carboxylates include zinc octoate and zinc stearate.

The rubber composition of the core 4 may include a filler for the purpose of specific gravity adjustment and the like. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is achieved.

The rubber composition of the core 4 may include various additives, such as sulfur, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like, in an adequate amount. The rubber composition may include crosslinked rubber powder or synthetic resin powder.

The core 4 preferably has a diameter D1 of not less than 35.0 mm and not greater than 40.7 mm. The golf ball 2 that includes the core 4 having a diameter D1 of not less than 35.0 mm has excellent resilience performance. From this viewpoint, the diameter D1 is more preferably not less than 37.0 mm and particularly preferably not less than 38.0 mm. The golf ball 2 that includes the core 4 having a diameter D1 of not greater than 40.7 mm has excellent durability. From this viewpoint, the diameter D1 is more preferably not greater than 40.3 mm and particularly preferably not greater than 40.0 mm.

The core 4 preferably has a central hardness H1o of not less than 45 and not greater than 70. The golf ball 2 that includes the core 4 having a central hardness H1o of not less than 45 has excellent resilience performance. From this viewpoint, the central hardness H1o is more preferably not less than 48 and particularly preferably not less than 50. The golf ball 2 that includes the core 4 having a central hardness H1o of not greater than 70 has excellent feel at impact. From this viewpoint, the central hardness H1o is more preferably not greater than 67 and particularly preferably not greater than 65.

The central hardness H1o is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against the central point of the cross-section of a hemisphere obtained by cutting the golf ball 2. The measurement is conducted in an environment of 23° C.

A hardness H1₍₁₀₎ of the core 4 at a point at which a distance from the central point of the core 4 is 10 mm is preferably not less than 55 and not greater than 85. The golf ball 2 that includes the core 4 having a hardness H1₍₁₀₎ of not less than 55 has excellent resilience performance. From this viewpoint, the hardness H1₍₁₀₎ is more preferably not less than 58 and particularly preferably not less than 60. The golf ball 2 that includes the core 4 having a hardness H1₍₁₀₎ of not greater than 85 has excellent feel at impact. From this viewpoint, the hardness H1₍₁₀₎ is more preferably not greater than 82 and particularly preferably not greater than 80.

The hardness H1₍₁₀₎ is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against a position away from the central point by 10 mm on the cross-section of a hemisphere obtained by cutting the golf ball 2. The measurement is conducted in an environment of 23° C.

The core 4 preferably has a surface hardness H1s of not less than 70 and not greater than 90. The golf ball 2 that includes the core 4 having a surface hardness H1s of not less than 70 has excellent resilience performance. From this viewpoint, the surface hardness H1s is more preferably not less than 73 and particularly preferably not less than 75. The golf ball 2 that includes the core 4 having a surface hardness H1s of not greater than 90 has excellent feel at impact. From this viewpoint, the surface hardness H1s is more preferably not greater than 87 and particularly preferably not greater than 85.

The surface hardness H1s is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). The hardness scale is pressed against the surface of the core 4. The measurement is conducted in an environment of 23° C.

The difference (H1s-H1o) between the surface hardness H1s and the central hardness H1o is preferably not less than 14. The golf ball 2 that includes the core 4 having a difference (H1s-H1o) of not less than 14 has excellent flight performance upon shots with a driver and a middle iron. The golf ball 2 further has excellent feel at impact upon a driver shot and upon putting. From these viewpoints, the difference (H1s-H1o) is more preferably not less than 16 and particularly preferably not less than 18. From the viewpoint of durability of the golf ball 2, the difference (H1s-H1o) is preferably not greater than 30.

The difference (H1s-H1₍₁₀₎) between the surface hardness H1s and the hardness H1₍₁₀₎ at the point at which the distance from the central point of the core 4 is 10 mm is preferably not less than 7. The golf ball 2 that includes the core 4 having a difference (H1s-H1₍₁₀₎) of not less than 7 has low spin upon a shot with a driver and therefore has excellent flight performance. The golf ball 2 further has excellent feel at impact upon a driver shot and upon putting. From these viewpoints, the difference (H1s-H1₍₁₀₎) is more preferably not less than 8 and particularly preferably not less than 9. From the viewpoint of durability of the golf ball 2, the difference (H1s-H1₍₁₀₎) is preferably not greater than 15.

The core 4 preferably has a weight of not less than 10 g and not greater than 42 g. The temperature for crosslinking the core 4 is typically not lower than 140° C. and not higher than 180° C. The time period for crosslinking the core 4 is typically not shorter than 10 minutes and not longer than 60 minutes.

The mid layer 6 is positioned outside the core 4. The mid layer 6 is formed from a thermoplastic resin composition. Examples of the base polymer of the resin composition include ionomer resins, thermoplastic polyester elastomers, thermoplastic polyamide elastomers, thermoplastic polyurethane elastomers, thermoplastic polyolefin elastomers, and thermoplastic polystyrene elastomers. Ionomer resins are particularly preferable. Ionomer resins are highly elastic. The golf ball 2 that includes the mid layer 6 including an ionomer resin has excellent resilience performance. The golf ball 2 has excellent flight performance.

An ionomer resin and another resin may be used in combination. In this case, from the viewpoint of resilience performance, the ionomer resin is included as the principal component of the base polymer. The proportion of the ionomer resin to the entire base polymer is preferably not less than 50% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or more but 90% by weight or less of an α-olefin, and 10% by weight or more but 20% by weight or less of an α ,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α, β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α, β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or more but 85% by weight or less of an α-olefin, 5% by weight or more but 30% by weight or less of an α, β-unsaturated carboxylic acid, and 1% by weight or more but 25% by weight or less of an α, βunsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymer and the ternary copolymer, preferable α-olefins are ethylene and propylene, while preferable α, β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. A particularly preferable ionomer resin is a copolymer formed with ethylene and acrylic acid. Another particularly preferable ionomer resin is a copolymer formed with ethylene and methacrylic acid.

In the binary copolymer and the ternary copolymer, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ions, potassium ions, lithium ions, zinc ions, calcium ions, magnesium ions, aluminum ions, and neodymium ions. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions from the viewpoint of resilience performance and durability of the golf ball 2 are sodium ions, zinc ions, lithium ions, and magnesium ions.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7329”, and “Himilan AM7337”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Corporation. Two or more ionomer resins may be used in combination.

The resin composition of the mid layer 6 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like in an adequate amount. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The mid layer 6 preferably has a thickness T2 of not less than 0.5 mm and not greater than 2.0 mm. The golf ball 2 in which the thickness T2 is not less than 0.5 mm has excellent resilience performance. From this viewpoint, the thickness T2 is more preferably not less than 0.7 mm and particularly preferably not less than 0.9 mm. The golf ball 2 in which the thickness T2 is not greater than 2.0 mm has excellent feel at impact. From this viewpoint, the thickness T2 is preferably not greater than 1.7 mm and particularly preferably not greater than 1.5 mm. The thickness T2 is measured at a position immediately below the land 16.

The mid layer 6 preferably has a material hardness H2 of not less than 55 and not greater than 75. The golf ball 2 in which the hardness H2 is not less than 55 has excellent resilience performance. From this viewpoint, the hardness H2 is more preferably not less than 58 and particularly preferably not less than 60. The golf ball 2 in which the hardness H2 is not greater than 75 has excellent feel at impact. From this viewpoint, the hardness H2 is more preferably not greater than 72 and particularly preferably not greater than 70.

The hardness H2 of the mid layer 6 is measured according to the standards of “ASTM-D 2240-68”. The hardness H2 is measured with a Shore D type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss Prüfgerätebau GmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the mid layer 6, and has a thickness of about 2 mm, is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the time of measurement, three sheets are stacked.

The mid layer 6 preferably has a surface hardness H2s of not less than 85 and not greater than 100. The golf ball 2 in which the surface hardness H2s is not less than 85 has excellent resilience performance. From this viewpoint, the surface hardness H2s is more preferably not less than 88 and particularly preferably not less than 90. The golf ball 2 in which the surface hardness H2s is not greater than 100 has excellent feel at impact. From this viewpoint, the surface hardness H2s is more preferably not greater than 98 and particularly preferably not greater than 96.

The surface hardness H2s is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss PrüfgerätebauGmbH). The hardness scale is pressed against the surface of a sphere consisting of the core 4 and the mid layer 6. The measurement is conducted in an environment of 23° C.

The cover 8 is formed from a resin composition. Examples of the base polymer of the resin composition include polyurethanes, ionomer resins, polyesters, polyamides, polyolefins, and polystyrenes. A preferable base polymer from the viewpoint of feel at impact and spin performance is a polyurethane. When a polyurethane and another resin are used in combination for the cover 8, the proportion of the polyurethane to the entire base resin is preferably not less than 50% by weight, more preferably not less than 60% by weight, and particularly preferably not less than 70% by weight.

The resin composition of the cover 8 may include a thermoplastic polyurethane or may include a thermosetting polyurethane. From the viewpoint of productivity of the golf ball 2, the thermoplastic polyurethane is preferable. The thermoplastic polyurethane includes a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment. The thermoplastic polyurethane is flexible. The cover 8 in which the polyurethane is used has excellent scuff resistance.

The thermoplastic polyurethane has a urethane bond within the molecule. The urethane bond can be formed by reacting a polyol with a polyisocyanate. The polyol, as a material for the urethane bond, has a plurality of hydroxyl groups. Low-molecular-weight polyols and high-molecular-weight polyols can be used.

Examples of low-molecular-weight polyols include diols, triols, tetraols, and hexaols. Specific examples of diols include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2,3-dimethyl-2,3-butanediol, neopentyl glycol, pentanediol, hexanediol, heptanediol, octanediol, and 1,6-cyclohexanedimethylol. Aniline diols or bisphenol A diols may be used. Specific examples of triols include glycerin, trimethylol propane, and hexanetriol. Specific examples of tetraols include pentaerythritol and sorbitol.

Examples of high-molecular-weight polyols include: polyether polyols such as polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polytetramethylene ether glycol (PTMG); condensed polyester polyols such as polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA); lactone polyester polyols such as poly-ε-caprolactone (PCL); polycarbonate polyols such as polyhexamethylene carbonate; and acrylic polyols. Two or more polyols may be used in combination. From the viewpoint of feel at impact of the golf ball 2, the high-molecular-weight polyol has a number average molecular weight of preferably not less than 400 and more preferably not less than 1000. The number average molecular weight is preferably not greater than 10000.

Examples of polyisocyanates, as a material for the urethane bond, include aromatic diisocyanates, alicyclic diisocyanates, and aliphatic diisocyanates. Two or more types of diisocyanates may be used in combination.

Examples of aromatic diisocyanates include 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 4,4’-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3’-bitolylene-4,4’-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI). One example of aliphatic diisocyanates is hexamethylene diisocyanate (HDI). Examples of alicyclic diisocyanates include 4,4’-dicyclohexylmethane diisocyanate (H₁₂MDI), 1,3-bis(isocyanatemethyl)cyclohexane (H₆XDI), isophorone diisocyanate (IPDI), and trans-1,4-cyclohexane diisocyanate (CHDI). 4,4’-dicyclohexylmethane diisocyanate is preferable.

Specific examples of the thermoplastic polyurethane include trade names “Elastollan NY80A”, “Elastollan NY82A”, “Elastollan NY83A”, “Elastollan NY84A”, “Elastollan NY85A”, “Elastollan NY88A”, “Elastollan NY90A”, “Elastollan NY95A”, “Elastollan NY97A”, “Elastollan NY585”, and “Elastollan KP016N”, manufactured by BASF Japan Ltd.; and trade names “RESAMINE P4585LS” and “RESAMINE PS62490”, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

The resin composition of the cover 8 may include a coloring agent, a filler, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, a lubricant, and the like in an adequate amount. When the hue of the golf ball 2 is white, a typical coloring agent is titanium dioxide.

The cover 8 preferably has a thickness T3 of not less than 0.3 mm and not greater than 1.5 mm. The golf ball 2 in which the thickness T3 is not less than 0.3 mm has excellent controllability and feel at impact. From this viewpoint, the thickness T3 is more preferably not less than 0.4 mm and particularly preferably not less than 0.5 mm. The golf ball 2 in which the thickness T3 is not greater than 1.5 mm has excellent resilience performance. From this viewpoint, the thickness T3 is more preferably not greater than 1.0 mm and particularly preferably not greater than 0.8 mm. The thickness T3 is measured at a position immediately below the land 16.

The cover 8 preferably has a material hardness H3 of not less than 20 and not greater than 45. The golf ball 2 in which the hardness H3 is not less than 20 has excellent durability. From this viewpoint, the hardness H3 is more preferably not less than 25 and particularly preferably not less than 28. The golf ball 2 in which the hardness H3 is not greater than 45 has excellent controllability and feel at impact. From this viewpoint, the hardness H3 is more preferably not greater than 40 and particularly preferably not greater than 35.

The hardness H3 of the cover 8 is measured according to the standards of “ASTM-D 2240-68”. The hardness H3 is measured with a Shore D type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss PrüfgerätebauGmbH). For the measurement, a sheet that is formed by hot press, is formed from the same material as that of the cover 8, and has a thickness of about 2 mm, is used. Prior to the measurement, a sheet is kept at 23° C. for two weeks. At the time of measurement, three sheets are stacked.

The cover 8 preferably has a surface hardness H3s of not less than 70 and not greater than 95. The golf ball 2 in which the surface hardness H3s is not less than 70 has excellent resilience performance. From this viewpoint, the surface hardness H3s is more preferably not less than 75 and particularly preferably not less than 80. The golf ball 2 in which the surface hardness H3s is not greater than 95 has excellent feel at impact. From this viewpoint, the surface hardness H3s is more preferably not greater than 92 and particularly preferably not greater than 90.

The surface hardness H3s is measured with a Shore C type hardness scale mounted to an automated hardness meter (trade name “digi test II” manufactured by Heinrich Bareiss PrüfgerätebauGmbH). The hardness scale is pressed against the surface of a sphere consisting of the core 4, the mid layer 6, and the cover 8. The measurement is conducted in an environment of 23° C.

The golf ball 2 may include a reinforcing layer between the mid layer 6 and the cover 8. The reinforcing layer firmly adheres to the mid layer 6 and also to the cover 8. The reinforcing layer suppresses separation of the cover 8 from the mid layer 6. The reinforcing layer is formed from a resin composition. Examples of a preferable base polymer of the reinforcing layer include two-component curing type epoxy resins and two-component curing type urethane resins. The reinforcing layer preferably has a thickness of not less than 5 µm and not greater than 30 µm.

The ratio ((T2+T3)/D1) of the sum (T2+T3) of the thickness T2 of the mid layer 6 and the thickness T3 of the cover 8 to the diameter D1 of the core 4 is preferably not greater than 0.042. The golf ball 2 in which this ratio is not greater than 0.042 has excellent flight performance upon shots with a driver and a middle iron. From this viewpoint, the ratio ((T2+T3)/D1) is more preferably not greater than 0.040 and particularly preferably not greater than 0.038. From the viewpoint of durability of the golf ball 2, the ratio ((T2+T3)/D1) is preferably not less than 0.032.

The ratio (H2/H3) of the hardness H2 of the mid layer 6 to the hardness H3 of the cover 8 is preferably not less than 1.80. The golf ball 2 in which the ratio (H2/H3) is not less than 1.80 has excellent flight performance upon shots with a driver and a middle iron. The golf ball 2 further has excellent feel at impact upon putting. From these viewpoints, the ratio (H2/H3) is more preferably not less than 2.00 and particularly preferably not less than 2.10. The ratio (H2/H3) is preferably not greater than 3.00.

Preferably, the surface hardness H2s of the mid layer 6 is greater than the surface hardness H1s of the core 4 and is greater than the surface hardness H3s of the cover 8. The golf ball 2 has excellent flight performance upon shots with a driver and a middle iron. The golf ball 2 further has excellent feel at impact upon a driver shot and upon putting. From these viewpoints, the difference (H2s-H1s) is preferably not less than 6, more preferably not less than 9, and particularly preferably not less than 11. The difference (H2s-H1s) is preferably not greater than 20. The difference (H2s-H3s) is preferably not less than 3, more preferably not less than 5, and particularly preferably not less than 7. The difference (H2s-H3s) is preferably not greater than 15.

The inner paint layer 10 is formed from a resin composition. Examples of the base resin of the resin composition include urethane resins, epoxy resins, acrylic resins, vinyl acetate resins, and polyester resins. Particularly preferable base resins are urethane resins. The inner paint layer 10 preferably has a thickness of not less than 5 µm and not greater than 20 µm.

The outer paint layer 12 is formed from a resin composition. Examples of the base resin of the resin composition include urethane resins, epoxy resins, acrylic resins, vinyl acetate resins, and polyester resins. Particularly preferable base resins are urethane resins. The outer paint layer 12 preferably has a thickness of not less than 5 µm and not greater than 20 µm.

Typically, the inner paint layer 10 is formed from a polyurethane paint, and the outer paint layer 12 is formed from another polyurethane paint. A preferable polyurethane paint includes a base material and a curing agent. The base material is a polyol composition (A), and the curing agent is a polyisocyanate composition (B).

The polyol composition (A) contains a polyol compound. The polyol compound has two or more hydroxyl groups within the molecule thereof. The polyol compound may be a polyol compound (a1) having a hydroxyl group at an end of the molecular chain thereof, or may be a polyol compound (a2) having a hydroxyl group at a portion of the molecular chain other than the ends thereof. The polyol composition (A) may contain two or more polyol compounds.

Examples of the polyol compound (a1) having a hydroxyl group at an end of the molecular chain thereof include: diols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; and triols such as glycerin, trimethylol propane, and hexanetriol. Other examples of the polyol compound (a1) having a hydroxyl group at an end of the molecular chain thereof include: polyether polyols, polyester polyols, polycaprolactone polyols, polycarbonate polyols, urethane polyols, and acrylic polyols. Examples of polyether polyols include polyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG). Examples of polyester polyols include polyethylene adipate diol, polybutylene adipate diol, and polyhexamethylene adipate diol. Examples of polycaprolactone polyols include poly-s-caprolactone diol. Examples of polycarbonate polyols include polyhexamethylene carbonate diol.

Examples of the polyol compound (a2) having a hydroxyl group at the portion of the molecule other than the ends thereof include a modified polyrotaxane having a hydroxyl group, and a hydroxyl group-modified vinyl chloride-vinyl acetate copolymer.

The polyisocyanate composition (B) contains a polyisocyanate compound. The polyisocyanate compound has two or more isocyanate groups. Examples of the polyisocyanate compound include: aromatic diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, a mixture (TDI) of 2,4-toluene diisocyanate and 2,6-toluene diisocyanate, 4,4’-diphenylmethane diisocyanate (MDI), 1,5-naphthylene diisocyanate (NDI), 3,3’-bitolylene-4,4’-diisocyanate (TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), and paraphenylene diisocyanate (PPDI); alicyclic or aliphatic diisocyanates such as 4,4’-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenated xylylene diisocyanate (H₆XDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), and norbornene diisocyanate (NBDI); and triisocyanates such as an allophanate product, a biuret product, an isocyanurate product, and an adduct product of diisocyanates. The polyisocyanate composition (B) may include two or more polyisocyanate compounds.

The outer paint layer 12 preferably has an indentation depth of not less than 300 nm. The golf ball 2 that includes the outer paint layer 12 having an indentation depth of not less than 300 nm has excellent feel at impact upon a driver shot and upon putting. From this viewpoint, the indentation depth is more preferably not less than 500 nm and particularly preferably not less than 700 nm. From the viewpoint of durability of the outer paint layer 12, the indentation depth is preferably not greater than 3000 nm.

In measurement of the indentation depth, the golf ball 2 is divided to obtain a hemisphere. On the hemisphere, a cross-section passing through the central point of the golf ball 2 is exposed. The cross-section includes a cross-section of the outer paint layer 12. The cross-section of the hemisphere is made horizontal by a cryo-microtome. A penetrator of a nanoindenter is brought into contact with the cross-section of the outer paint layer 12 and pressed against the cross-section in a direction perpendicular to the cross-section. Due to this pressing, the penetrator advances. The pressing is continued until the load of the penetrator reaches 50 mgf. The advancing distance of the penetrator when the load of the penetrator is 30 mgf is measured as an indentation depth. The measurement conditions are as follows.

-   Nanoindenter: “ENT-2100” manufactured by ELIONIX INC. -   Temperature: 30° C. -   Penetrator: Berkovich penetrator (65.03° As(h) = 26.43 h2) -   Number of partitions: 500 steps -   Step interval: 20 msec (100 mgf)

The golf ball 2 preferably has an amount of compressive deformation CD of not less than 3.00 mm. The golf ball 2 having an amount of compressive deformation CD of not less than 3.00 mm has excellent feel at impact upon a driver shot and upon putting. From this viewpoint, the amount of compressive deformation CD is more preferably not less than 3.15 mm and particularly preferably not less than 3.25 mm. From the viewpoint of flight performance upon shots with a driver and a middle iron, the amount of compressive deformation CD is preferably not greater than 4.00 mm.

The amount of compressive deformation CD is measured with a YAMADA type compression tester “SCH”. In the measurement, the golf ball 2 is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the golf ball 2. The golf ball 2, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the golf ball 2 up to the state in which a final load of 1275 N is applied thereto, is measured. A moving speed of the cylinder until the initial load is applied is 0.83 mm/s. A moving speed of the cylinder after the initial load is applied until the final load is applied is 1.67 mm/s.

In the present specification, a maximum vertical force of the golf ball 2 is measured under a first condition and a second condition. In the present specification, a maximum vertical force of a standard ball is also measured under the first condition and the second condition.

The specifications of the standard ball are as follows:

-   Core     -   Diameter: 39.0 mm     -   Central hardness: 62 (Shore C)     -   Hardness at point at which distance from central point is 10 mm:         71 (Shore C)     -   Surface hardness: 78 (Shore C)

    Mid layer     -   Thickness: 1.4 mm     -   Material hardness: 71 (Shore D)     -   Surface hardness: 97 (Shore C)

    Cover     -   Thickness: 0.5 mm     -   Material hardness: 40 (Shore D)     -   Surface hardness: 87 (Shore C)

    Paint layer     -   Thickness: 10 µm     -   Indentation depth: 971 nm     -   Amount of compressive deformation CDn: 3.12 mm

In production of the standard ball, a rubber composition is obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-60”, manufactured by JSR Corporation), 24.3 parts by weight of zinc acrylate, 5.0 parts by weight of zinc oxide, 12.4 parts by weight of barium sulfate, and 0.6 parts by weight of dicumyl peroxide (DCP). This rubber composition is placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated to obtain a core with a diameter of 39.0 mm. A resin composition is obtained by kneading 50 parts by weight of an ionomer resin (the aforementioned “Surlyn 8150”), 50 parts by weight of another ionomer resin (the aforementioned “Surlyn 9150”), 4 parts by weight of titanium dioxide, and 9 parts by weight of barium sulfate with a twin-screw kneading extruder. The core is placed into a mold including upper and lower mold halves each having a hemispherical cavity. The core is covered with the resin composition by injection molding to form a mid layer. The thickness of the mid layer is 1.4 mm. A resin composition is obtained by kneading the aforementioned PTMG and TDI at a ratio that can achieve a Shore D hardness of 40. A final mold including upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face is heated to a temperature of not lower than 80° C. and not higher than 100° C. The resin composition is placed into the final mold, the sphere including the core and the mid layer is further placed thereinto, and a cover is formed by compression molding. The thickness of the cover is 0.5 mm. Dimples having a shape that is the inverted shape of the pimples are formed on the cover. A clear paint including a two-component curing type polyurethane as a base material is applied to the cover. The paint is dried to obtain a standard golf ball.

In the first condition, the golf ball 2 or the standard ball is hit with a driver 18 (W#1, trade name “XXIO 11”, manufactured by Sumitomo Rubber Industries, Ltd.) shown in FIGS. 2 and 3 . FIG. 3 also shows an acceleration sensor 20 (trade name “#4393”, manufactured by B&K). The driver 18 has a head 22 and a shaft 24. The loft angle of the head 22 is 10.5°, and the hardness of the shaft 24 is S. The product number of the shaft 24 is MP1100. The head 22 includes a face 26, a crown 28, and a sole 30. The face 26 has a toe-side end point 32, a heel-side end point 34, a top edge line 36, and a bottom edge line 38. In FIGS. 2 and 3 , reference character CP represents the central point of the face 26. The central point CP is on a center line CL. The distance from the center line CL to the heel-side end point 34 is equal to the distance from the center line CL to the toe-side end point 32. The distance from the central point CP to the point of intersection of the center line CL and the bottom edge line 38 is equal to the distance from the central point CP to the point of intersection of the center line CL and the top edge line 36. A virtual line L1 shown in FIG. 3 passes through the central point CP and is orthogonal to the face 26. At the point of intersection of the virtual line L1 and the outer surface of the sole 30, the acceleration sensor 20 is fixed to the sole 30.

The driver 18 is attached to a swing machine (Golf Laboratories, Inc.). The golf ball 2 or the standard ball is hit with the driver 18 under the condition of a head speed of 40 m/s. The hitting point in this hitting is the central point CP. The acceleration in the hitting direction at the time of this hitting is measured by the acceleration sensor 20. The product of the total weight of the head 22 and the acceleration sensor 20 and the acceleration is a vertical force. The time from the time point at which the vertical force is generated to the time point at which the vertical force is eliminated is a contact time. The maximum value of the vertical force during the contact time is a maximum vertical force. The average value of data obtained by 12 hits is calculated.

In the second condition, the golf ball 2 or the standard ball is hit with a putter 40 (trade name “Huntington Beach Soft Putter #4”, manufactured by Cleveland Golf Company, Inc.) shown in FIGS. 4 and 5 . FIGS. 4 and 5 also show an acceleration sensor 42 (trade name “356A01”, manufactured by TOYO Corporation). The putter 40 has a head 44 and a shaft 46. The head 44 includes a face 48 and a sole 50. The face 48 has a toe-side end point 52, a heel-side end point 54, and a bottom edge line 56. In FIGS. 4 and 5 , reference character CP represents the central point of the face 48. The central point CP is on a center line CL. The distance from the center line CL to the heel-side end point 54 is equal to the distance from the center line CL to the toe-side end point 52. The distance from the bottom edge line 56 to the central point CP is 20 mm. On the back side of the central point CP, the acceleration sensor 42 is fixed to the head 44.

The putter 40 is attached to a swing machine (Golf Laboratories, Inc.). The golf ball 2 or the standard ball is hit with the putter 40 under a condition that a rolling distance is 15 m on a green that is horizontal and whose speed measured by a stimpmeter is 10.0 m. The hitting point in this hitting is the central point CP. The acceleration in the hitting direction at the time of this hitting is measured by the acceleration sensor 42. The product of the total weight of the head 44 and the acceleration sensor 42 and the acceleration is a vertical force. The time from the time point at which the vertical force is generated to the time point at which the vertical force is eliminated is a contact time. The maximum value of the vertical force during the contact time is a maximum vertical force. The average value of data obtained by 12 hits is calculated.

The golf ball 2 according to the present embodiment satisfies the following mathematical formulas (1) and (2).

(Fw/Fwn) ≤ 0.98

(Fw/Fwn) ² /  (Fp/Fpn)  ≤  0.98

Fw: the maximum vertical force of the golf ball 2 measured under the first condition.

Fwn: the maximum vertical force of the standard ball measured under the first condition.

Fp: the maximum vertical force of the golf ball 2 measured under the second condition.

Fpn: the maximum vertical force of the standard ball measured under the second condition.

Soft feel at impact is achieved upon a driver shot of the golf ball 2 that satisfies the above mathematical formula (1). With the golf ball 2 that satisfies the above mathematical formula (2), both soft feel at impact upon a driver shot and soft feel at impact upon putting are achieved. With the golf ball 2, flight performance upon shots with the driver 18 and a middle iron is not impaired.

In the golf ball 2 that satisfies the above mathematical formula (1), the ratio (Fw/Fwn) is not greater than 0.98. From the viewpoint of feel at impact upon a driver shot, the ratio (Fw/Fwn) is more preferably not greater than 0.96 and particularly preferably not greater than 0.94. The ratio (Fw/Fwn) is preferably not less than 0.85.

In the golf ball 2 that satisfies the above mathematical formula (2), the ratio ((Fw/Fwn)²/(Fp/Fpn)) is not greater than 0.98. From the viewpoint of feel at impact upon a driver shot and upon putting, the ratio ((Fw/Fwn)²/(Fp/Fpn)) is more preferably not greater than 0.96 and particularly preferably not greater than 0.95. The ratio ((Fw/Fwn)²/(Fp/Fpn)) is preferably not less than 0.85.

Examples of means for achieving the above mathematical formulas (1) and (2) include

-   (a) setting the hardness difference (H1s-H1o) of the core 4 to be     large, -   (b) setting the hardness difference (H1s-H1₍₁₀₎) of the core 4 to be     large, -   (c) setting the ratio (H2/H3) of the hardness H2 of the mid layer 6     to the hardness H3 of the cover 8 to be large, -   (d) setting the ratio of the sum (T2+T3) of the thickness T2 of the     mid layer 6 and the thickness T3 of the cover 8 to the diameter D1     of the core 4 to be small, and the like.

Preferably, the golf ball 2 satisfies the following mathematical formula (3).

(Fp/Fpn)  ≤  0.98

In other words, the ratio (Fp/Fpn) is not greater than 0.98. With the golf ball 2, both soft feel at impact and firm feel are achieved upon putting. From this viewpoint, the ratio (Fp/Fpn) is more preferably not greater than 0.96 and particularly preferably not greater than 0.95. The ratio (Fp/Fpn) is preferably not less than 0.80.

Preferably, the golf ball 2 satisfies the following mathematical formula (4).

(Tp/Tpn)  ≥  1.01

Tp: the contact time of the golf ball 2 measured under the second condition.

Tpn: the contact time of the standard ball measured under the second condition.

In other words, the ratio (Tp/Tpn) is not less than 1.01. With the golf ball 2, both soft feel at impact and firm feel are achieved upon putting. From this viewpoint, the ratio (Tp/Tpn) is more preferably not less than 1.02 and particularly preferably not less than 1.03. The ratio (Tp/Tpn) is preferably not greater than 1.15.

Preferably, the golf ball 2 satisfies the following mathematical formula (5).

(Fp/Fpn)  /  (Tp/Tpn)  ≤  0.98

In other words, the ratio ((Fp/Fpn)/(Tp/Tpn)) is not greater than 0.98. With the golf ball 2, both soft feel at impact and firm feel are achieved upon putting. From this viewpoint, the ratio ((Fp/Fpn)/(Tp/Tpn)) is more preferably not greater than 0.95 and particularly preferably not greater than 0.92. The ratio ((Fp/Fpn)/(Tp/Tpn)) is preferably not less than 0.75.

Preferably, the golf ball 2 satisfies the following mathematical formula (6).

(Tw/Twn) ≥ 1.01

Tw: the contact time of the golf ball 2 measured under the first condition.

Twn: the contact time of the standard ball measured under the first condition.

In other words, the ratio (Tw/Twn) is not less than 1.01. With the golf ball 2, both soft feel at impact and bite feel are achieved upon a driver shot. From this viewpoint, the ratio (Tw/Twn) is particularly preferably not less than 1.02. The ratio (Tw/Twn) is preferably not greater than 1.20.

Preferably, the golf ball 2 satisfies the following mathematical formula (7).

(Fw/Fwn)/(Tw/Twn) ≤ 0.98

In other words, the ratio ((Fw/Fwn)/(Tw/Twn)) is not greater than 0.98. With the golf ball 2, both soft feel at impact and bite feel are achieved upon a driver shot. From this viewpoint, the ratio ((Fw/Fwn)/(Tw/Twn)) is more preferably not greater than 0.95 and particularly preferably not greater than 0.92. The ratio ((Fw/Fwn)/(Tw/Twn)) is preferably not less than 0.75.

EXAMPLES

The following will show the advantageous effects of golf balls according to Examples, but the scope disclosed in the present specification should not be construed in a limited manner on the basis of the description of these Examples.

Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-60”, manufactured by JSR Corporation), 25.3 parts by weight of zinc diacrylate, 5.0 parts by weight of zinc oxide, 12.0 parts by weight of barium sulfate, 0.3 parts by weight of pentabromodiphenyl disulfide (PBDS), and 0.7 parts by weight of dicumyl peroxide (DCP). The rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated to obtain a core with a diameter of 39.7 mm.

A resin composition was obtained by kneading 50 parts by weight of an ionomer resin (the aforementioned “Himilan 1605”), 50 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 4 parts by weight of titanium dioxide, and 9 parts by weight of barium sulfate with a twin-screw kneading extruder. The core was placed into a mold including upper and lower mold halves each having a hemispherical cavity. The core was covered with the resin composition by injection molding to form a mid layer. The thickness of the mid layer was 1.0 mm.

A paint composition (trade name “POLIN 750LE”, manufactured by SHINTO PAINT CO., LTD.) including a two-component curing type epoxy resin as a base polymer was prepared. The base material liquid of this paint composition includes 30 parts by weight of a bisphenol A type epoxy resin and 70 parts by weight of a solvent. The curing agent liquid of this paint composition includes 40 parts by weight of a modified polyamide amine, 55 parts by weight of a solvent, and 5 parts by weight of titanium dioxide. The weight ratio of the base material liquid to the curing agent liquid is 1/1. This paint composition was applied to the surface of the mid layer with a spray gun, and kept at 23° C. for 12 hours to obtain a reinforcing layer. The thickness of the reinforcing layer was 10 µm.

A resin composition was obtained by kneading 100 parts by weight of a thermoplastic polyurethane elastomer (the aforementioned “Elastollan NY84A”), 2 parts by weight of a lubricant (trade name “Wax Master VD”, manufactured by BASF Japan Ltd.), 4 parts by weight of titanium dioxide, and 0.2 parts by weight of a light stabilizer (trade name “JF-90”, manufactured by Johoku Chemical Co., Ltd.) with a twin-screw kneading extruder. Half shells were obtained from this resin composition by compression molding. The sphere including the core, the mid layer, and the reinforcing layer was covered with two of these half shells. These half shells and the sphere were placed into a final mold including upper and lower mold halves each having a hemispherical cavity and having a large number of pimples on its cavity face, and a cover was obtained by compression molding. The thickness of the cover was 0.5 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the cover.

A clear paint including a two-component curing type polyurethane as a base material was applied to the cover to form an inner paint layer. Another clear paint including a two-component curing type polyurethane as a base material was applied to the inner paint layer to form an outer paint layer. Thus, a golf ball of Example 1 with a diameter of about 42.7 mm and a weight of about 45.3 g was obtained.

Example 2

A golf ball of Example 2 was obtained in the same manner as Example 1, except the specifications of the core, the mid layer, and the cover were as shown in Tables 1 and 2 below.

Comparative Examples 1 and 2

As Comparative Example 1, a commercially available golf ball was prepared. As Comparative Example 2, another commercially available golf ball was prepared.

Flight Test, W#1

A driver (trade name “XXIO 11”, manufactured by Sumitomo Rubber Industries, Ltd., shaft product number: MP1100, shaft hardness: S, loft angle: 10.5°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 40 m/sec, and the spin rate and the flight distance were measured. The flight distance is the distance from the hitting spot to the spot at which the golf ball stopped. The measurement was conducted 12 times, and the average value of the obtained data was calculated. Furthermore, the evaluation was categorized based on the average value of flight distance.

-   A: 215.5 yards or more -   B: Less than 215.5 yards

The results are shown in Table 3 below.

Flight Test, I#7

A 7-iron club (trade name “XXIO 11”, manufactured by Sumitomo Rubber Industries, Ltd., shaft product number: MP1100, shaft hardness: S, loft angle: 28.0°) was attached to a swing machine manufactured by Golf Laboratories, Inc. A golf ball was hit under the condition of a head speed of 33 m/sec, and the spin rate and the flight distance were measured. The measurement was conducted 12 times, and the average value of the obtained data was calculated. Furthermore, the evaluation was categorized based on the average value of flight distance.

-   A: 148.0 yards or more -   B: 145.0 yards or more and less than 148.0 yards -   C: Less than 145.0 yards

The results are shown in Table 3 below.

Feel at Impact, W#1

A golf ball was hit with a driver by a tester, and a score was given according to the following criteria.

-   0 points: Hard -   2 points: Slightly hard -   5 points: Normal -   8 points: Slightly soft -   10 points: Soft

The sum of scores by 50 testers was calculated, and the evaluation was categorized as follows.

-   A: 310 points or more -   B: 300 points or more and less than 310 points -   C: Less than 300 points

The results are shown in Table 3 below.

Feel at Impact, Putter

A golf ball was hit with a putter by a tester, and a score was given according to the following criteria.

-   0 points: Hard -   2 points: Slightly hard -   5 points: Normal -   8 points: Slightly soft -   10 points: Soft

The sum of scores by 50 testers was calculated, and the evaluation was categorized as follows.

-   A: 330 points or more -   B: 300 points or more and less than 330 points -   C: Less than 300 points

The results are shown in Table 3 below.

Table 1 Composition (parts by weight) Ex. 1 Ex. 2 Core BR 60 100 100 Zinc diacrylate 25.3 25.8 Zinc oxide 5.0 5.0 Barium sulfate 12.0 13.4 PBDS 0.3 0.3 DCP 0.7 0.7 Mid layer Himilan 1605 50 50 Himilan AM7329 50 50 Titanium dioxide 4 4 Barium sulfate 9 Cover Elastollan NY84A 100 100 Wax Master VD 2 2 Titanium dioxide 4 4 JF-90 0.2 0.2

Table 2 Specifications Ex. 1 Ex. 2 Comp. Ex. 1 Comp. Ex. 2 Standard ball Core D1 (mm) 39.7 39.7 39.3 39.1 39.0 H1o (Shore C) 56 61 66 62 62 H1 ₍₁₀₎ (Shore C) 69 70 73 74 71 H1s (Shore C) 81 83 81 74 78 Mid layer T2 (mm) 1.0 1.0 0.7 1.0 1.4 H2 (Shore D) 66 65 66 68 71 H2s (Shore C) 95 94 95 97 97 Cover T3 (mm) 0.5 0.5 1.0 0.8 0.5 H3 (Shore D) 31 31 45 48 40 H3s (Shore C) 87 87 87 87 87 Outer paint layer Indentation depth (nm) 759 759 971 CD (mm) 3.33 3.26 2.82 2.88 3.12 H1s-H1o 25 22 15 12 11 (T2+T3)/D1 0.038 0.038 0.043 0.046 0.049 H2/H3 2.13 2.10 1.47 1.42 1.78

Table 3 Evaluation Results Ex. 1 Ex. 2 Comp. Ex. 1 Comp. Ex. 2 Standard ball Fw/ Fwn 0.93 0.94 1.08 1.09 1.00 Tw/Twn 1.09 1.02 0.92 0.93 1.00 (Fw/Fwn) / (Tw/Twn) 0.85 0.92 1.17 1.17 1.00 Fp/Fpn 0.90 0.95 1.02 1.06 1.00 Tp/Tpn 1.06 1.03 0.96 0.93 1.00 (Fp/Fpn) / (Tp/Tpn) 0.85 0.92 1.06 1.14 1.00 (Fw/Fwn)²/ (Fp/Fpn) 0.95 | 0.93 | 1.15 1.12 1.00 Flight test W#1 Flight distance (yds.) 215.8 215.8 215.4 216.0 215.5 Rank A A B A A Spin (rpm) 2315 2317 2293 2339 2453 Flight test I#7 Flight distance (yds.) 149.6 148.5 144.8 144.3 147.3 Rank A A C C B Spin (rpm) 4383 4516 4967 5028 4658 Feel at impact W#1 Score 300 314 297 294 303 Rank B A C C B Feel at impact putter Score 351 334 295 286 304 Rank A A C C B

As shown in Table 3, the golf ball of each Example has excellent flight performance upon a shot with a driver, excellent flight performance upon a shot with a middle iron, excellent feel at impact upon a shot with a driver, and excellent feel at impact upon putting. From the evaluation results, advantages of the golf ball are clear.

Disclosure Items

Each of the following items discloses a preferred embodiment.

Item 1

A golf ball including a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, the golf ball satisfying the following mathematical formulas (1) and (2),

(Fw/Fwn) ≤ 0.98

(Fw/Fwn)²/(Fp/Fpn) ≤ 0.98

wherein Fw represents a maximum vertical force of the golf ball measured under a first condition, Fwn represents a maximum vertical force of a standard ball measured under the first condition, Fp represents a maximum vertical force of the golf ball measured under a second condition, and Fpn represents a maximum vertical force of the standard ball measured under the second condition.

Item 2

The golf ball according to Item 1, wherein the golf ball satisfies the mathematical formula (3),

(Fp/Fpn) ≤ 0.98

Item 3

The golf ball according to Item 1 or 2, wherein the golf ball satisfies the mathematical formula (4),

(Tp/Tpn)  ≥  1.01

wherein Tp represents a contact time of the golf ball measured under the second condition, and Tpn represents a contact time of the standard ball measured under the second condition.

Item 4

The golf ball according to Item 3, wherein the golf ball satisfies the mathematical formula (5),

(Fp/Fpn)  /  (Tp/Tpn)  ≤  0.98

Item 5

The golf ball according to any one of Items 1 to 4, wherein the golf ball satisfies the mathematical formula (6),

(Tw/Twn)  ≥  1.01

wherein Tw represents a contact time of the golf ball measured under the first condition, and Twn represents a contact time of the standard ball measured under the first condition.

Item 6

The golf ball according to Item 5, wherein the golf ball satisfies the mathematical formula (7),

(Fw/fwn)  /  (Tw/Twn)  ≤  0.98

Item 7

The golf ball according to any one of Items 1 to 6, wherein an amount of compressive deformation CD of the golf ball measured under a condition that an initial load is 98 N and a final load is 1275 N is not less than 3.00 mm.

Item 8

The golf ball according to any one of Items 1 to 7, wherein a surface hardness H2s (Shore C) of the mid layer is greater than a surface hardness H1s (Shore C) of the core and is greater than a surface hardness H3s (Shore C) of the cover.

Item 9

The golf ball according to any one of Items 1 to 8, wherein a difference (H1s-H1o) between a surface hardness H1s (Shore C) and a central hardness H1o (Shore C) of the core is not less than 14.

Item 10

The golf ball according to any one of Items 1 to 9, wherein a ratio of a sum (T2+T3) of a thickness T2 of the mid layer and a thickness T3 of the cover to a diameter D1 of the core is not greater than 0.042.

Item 11

The golf ball according to any one of Items 1 to 10, wherein a ratio (H2/H3) of a hardness H2 (Shore D) of the mid layer to a hardness H3 (Shore D) of the cover is not less than 1.80.

Item 12

The golf ball according to any one of Items 1 to 11, further including one or more paint layers positioned outside the cover, wherein an indentation depth of an outermost paint layer when a load is 30 mgf is not less than 300 nm and not greater than 3000 nm.

The aforementioned golf ball is suitable for, for example, playing golf on golf courses and practicing at driving ranges. The above descriptions are merely illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core, a mid layer positioned outside the core, and a cover positioned outside the mid layer, the golf ball satisfying the following mathematical formulas (1) and (2), (Fw/Fwn) ≤ 0.98 (1), (Fw/Fwn)² / (Fp/Fpn) ≤ 0.98 (2), wherein Fw represents a maximum vertical force of the golf ball measured under a first condition, Fwn represents a maximum vertical force of a standard ball measured under the first condition, Fp represents a maximum vertical force of the golf ball measured under a second condition, and Fpn represents a maximum vertical force of the standard ball measured under the second condition.
 2. The golf ball according to claim 1, wherein the golf ball satisfies the mathematical formula (3), (Fp/Fpn) ≤ 0.98 (3) .
 3. The golf ball according to claim 1, wherein the golf ball satisfies the mathematical formula (4), (Tp/Tpn) ≥ 1.01 (4), wherein Tp represents a contact time of the golf ball measured under the second condition, and Tpn represents a contact time of the standard ball measured under the second condition.
 4. The golf ball according to claim 3, wherein the golf ball satisfies the mathematical formula (5), (Fp/Fpn) / (Tp/Tpn) ≤ 0.98 (5).
 5. The golf ball according to claim 1, wherein the golf ball satisfies the mathematical formula (6), (Tw/Twn) ≥ 1.01 (6), wherein Tw represents a contact time of the golf ball measured under the first condition, and Twn represents a contact time of the standard ball measured under the first condition.
 6. The golf ball according to claim 5, wherein the golf ball satisfies the mathematical formula (7), (Fw/Fwn) / (Tw/Twn) ≤ 0.98 (7) .
 7. The golf ball according to claim 1, wherein an amount of compressive deformation CD of the golf ball measured under a condition that an initial load is 98 N and a final load is 1275 N is not less than 3.00 mm.
 8. The golf ball according to claim 1, wherein a surface hardness H2s (Shore C) of the mid layer is greater than a surface hardness H1s (Shore C) of the core and is greater than a surface hardness H3s (Shore C) of the cover.
 9. The golf ball according to claim 1, wherein a difference (H1s-H1o) between a surface hardness H1s (Shore C) and a central hardness H1o (Shore C) of the core is not less than
 14. 10. The golf ball according to claim 1, wherein a ratio of a sum (T2+T3) of a thickness T2 of the mid layer and a thickness T3 of the cover to a diameter D1 of the core is not greater than 0.042.
 11. The golf ball according to claim 1, wherein a ratio (H2/H3) of a hardness H2 (Shore D) of the mid layer to a hardness H3 (Shore D) of the cover is not less than 1.80.
 12. The golf ball according to claim 1, further comprising one or more paint layers positioned outside the cover, wherein an indentation depth of an outermost paint layer when a load is 30 mgf is not less than 300 nm and not greater than 3000 nm. 